1. Field of the Invention
The invention relates in general to a method of inspecting wafer water marks, and more particularly, to a method of inspecting wafer water marks using a patterned wafer.
2. Description of the Related Art
Typically, to ensure the quality of a wafer, a water mark formed during the process in a chemical station is inspected. For example, in a fabrication process with a sub-micron line width. before forming a gate oxide, a cleaning process is performed to ensure the quality of the gate oxide. This cleaning process greatly affects, or even determines the quality of the gate oxide layer formed subsequently. The cleaning process typically has the functions of removing particles, native oxide layer, organic chemicals, and metal contamination. During the cleaning process, in case of improperly controlling the technique of delivering or cleaning/drying, a water mark problem is caused.
In the example of forming a gate oxide layer, it is known that the quality of the gate oxide layer is a key factor which determines the performance of a metal-oxide semiconductor (MOS) device. When a gate oxide contains too much contamination or too many charges, the threshold voltage of the MOS device is altered. The breakdown voltage is decreased, and the lifetime is shortened.
In a conventional method of inspecting a water mark, a bare wafer or a wafer having a thin film without being patterned is provided for a test specimen. The defects of the wafer is evaluated or monitored by scanning the water mark by an inspecting machine such as a KLA/TENCOR machine, an optical microscope (OM) machine, or an atomic force microscope (AFM) machine. However, these kinds of machines can not effectively distinguish which kinds of defects are caused by particles caused by water marks. Generally speaking, the practical products commonly comprise a pattern, so that an obvious topography is shown. Thus, the process window of a patterned wafer is smaller than a bare wafer. As a consequence, the defects detected from the test specimen, that is, the inspection from the water mark of a bare wafer does not directly reflects the defects formed on a real products. On the other hand, the test wafer being processed can not be used again.
The process flow of a conventional method of inspecting water wafer marks is shown in FIG. 1. As shown as step 10, a bare wafer is provided. Or alternatively, a wafer having a thin film thereon without being patterned is provided. The wafer is used as a test specimen for water mark inspection.
In step 12, the wafer is disposed into a chemical station to form a water mark. The water mark is formed by simulating process conditions of a real wafer product.
In step 14, a defect inspection is performed to the test specimen, for example, the inspection to a water mark formed in the previous step.
In step 16, according to the water mark obtained in the test specimen the performance of a real wafer being processed in the chemical station is presumed.
FIG. 2 shows a process flow of another conventional method of inspecting a water mark. In step 20, a bare wafer or a wafer deposited with a thin film without being patterned is provided as a test specimen. In step 22, the wafer is disposed in a chemical station to form a water mark. The water mark is formed by simulating the process conditions for forming a real wafer product. In step 24, a thin film is formed on the wafer to emphasize the water mark formed in the chemical station. In step 26, a defect inspection is performed to the test specimen, for example, the inspection to a water mark formed in the previous step. In step 28, according to the water mark obtained in the test specimen, the performance of a wafer being processed in the chemical station is evaluated.
Since the real wafer products of wafers comprise patterns, an obvious surface topography is shown. However, in the conventional method of inspecting a water mark, a bare wafer or a wafer having a thin film formed thereon is used as a test specimen. Both of the bare wafer and the latter wafer do not have a pattern thereon. Even the wafer mark is formed by simulating the water mark of a real wafer product, the defects detected by inspecting the water mark cannot not precisely reflect the defects of the real wafer product. The test specimen can only be used to monitor the performance of a real wafer once, the used wafers cannot be recycled, and therefore, increases the inspection cost.